This proposal is designed to delineate and characterize the regulatory mechanism involved in the control of murine immunoglobulin class expression employing B cell cloning assays that facilitate the class switch and restricted or cloned helper T cell populations. Initially, we will continue to examine the role of the B cell in determining isotype expression by analyzing the extent to which B cells exhibit class commitment. This will be done by reducing problems associated with B cell heterogeneity through the separation of primed and unprimed B cells into defined B cell subsets based on s-Ig (e.g. s-IgA, s-IgE), Lyb surface antigens, and Fc receptors. In addition, we will examine the potential for isotype expression of these defined B cell subsets depending upon the nature of the stimulus by using T-dependent and T-independent forms of the antigen. We will then explore the functional relationship between these defined B cell subsets and helper T cells that are involved in the regulation of isotype expression. The parasite-infected mouse model provides a biological system whereby isotypic regulation by T cells may be maximal since these mice produce abnormally high levels of IgE. We will characterize the helper T cell population from such mice with particular emphasis on helper T cells that may be functionally restricted for the IgE isotype. The most powerful approach for determining the role of T cells in the class switch is the use of clonally derived helper T cells. We intend to clone helper T cells to a variety of carriers including parasite antigens and use these lines in B cell cloning assays with defined B cell subsets. Moreover, employing enrichment procedures we will try to generate isotype-specific T cells from both antigen-primed and parasite-infected mice. We will then define the nature of the target B cells as well as possible recognition elements involved when these functionally-restricted T cells are operative. The proposed studies should substantially contribute to our knowledge of the control mechanisms involved in immunoglobulin class production.